System and method for a MEMS device

ABSTRACT

According to an embodiment, a microelectromechanical systems MEMS device includes a first membrane attached to a support structure that a first plurality of acoustic vents; a second membrane attached to the support structure that includes a second plurality of acoustic vents, where the first plurality of acoustic vents and the second plurality of acoustic vents do not overlap; and a closing mechanism coupled to the first membrane and the second membrane.

This application is a continuation of U.S. patent application Ser. No.14/992,615, filed on Jan. 11, 2016, which application is herebyincorporated herein by reference in its entirety.

TECHNICAL FIELD

The present invention relates generally to transducers, and, inparticular embodiments, to a system and method for a MEMS device.

BACKGROUND

Transducers convert signals from one domain to another and are oftenused as sensors. For example, acoustic transducers convert betweenacoustic signals and electrical signals. A microphone is one type ofacoustic transducer that converts sound waves, i.e., acoustic signals,into electrical signals, and a speaker is one type of acoustictransducer that converts electrical signals into sound waves.

Microelectromechanical system (MEMS) based transducers include a familyof transducers produced using micromachining techniques. Some MEMStransducers, such as a MEMS microphone, gather information from theenvironment by measuring the change of physical state in the transducerand transferring the signal to be processed by the electronics which areconnected to the MEMS sensor. Other MEMS transducers, such as a MEMSmicrospeaker, convert electrical signals into a change in the physicalstate in the transducer. MEMS devices may be manufactured usingmicromachining fabrication techniques similar to those used forintegrated circuits.

MEMS devices may be designed to function as oscillators, resonators,accelerometers, gyroscopes, pressure sensors, microphones,micro-mirrors, microspeakers, etc. Many MEMS devices use capacitivesensing or actuation techniques for transducing the physical phenomenoninto electrical signals and vice versa. In such applications, thecapacitance change in the transducer is converted to a voltage signalusing interface circuits or a voltage signal is applied to thecapacitive structure in the transducer in order to generate a forcebetween elements of the capacitive structure.

For example, a capacitive MEMS microphone includes a backplate electrodeand a membrane arranged in parallel with the backplate electrode. Thebackplate electrode and the membrane form a parallel plate capacitor.The backplate electrode and the membrane are supported by a supportstructure arranged on a substrate.

The capacitive MEMS microphone is able to transduce sound pressurewaves, for example speech, at the membrane arranged in parallel with thebackplate electrode. The backplate electrode is perforated such thatsound pressure waves pass through the backplate while causing themembrane to vibrate due to a pressure difference formed across theforeside and backside of the membrane. Hence, the air gap between themembrane and the backplate electrode varies with vibrations of themembrane. The variation of the membrane in relation to the backplateelectrode causes variation in the capacitance between the membrane andthe backplate electrode. This variation in the capacitance istransformed into an output signal responsive to the movement of themembrane and forms a transduced signal.

Using a similar structure, a voltage signal may be applied between themembrane and the backplate in order to cause the membrane to vibrate andgenerate pressure pulses, such as sound pressure waves. Thus, acapacitive plate MEMS structure may operate as a microspeaker.

SUMMARY

According to an embodiment, According to an embodiment, amicroelectromechanical systems MEMS device includes a first membraneattached to a support structure that a first plurality of acousticvents; a second membrane attached to the support structure that includesa second plurality of acoustic vents, where the first plurality ofacoustic vents and the second plurality of acoustic vents do notoverlap; and a closing mechanism coupled to the first membrane and thesecond membrane. Other embodiments include corresponding systems andapparatus, each configured to perform various embodiment methods.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present invention, and theadvantages thereof, reference is now made to the following descriptionstaken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a system block diagram of an embodiment variable flowtransducer;

FIGS. 2A and 2B illustrate waveform diagrams of illustrative acousticsignals;

FIGS. 3A, 3B, and 3C illustrate side view cross-sections of anembodiment variable flow transducer;

FIGS. 4A, 4B, and 4C illustrate an embodiment model variable flowtransducer and a corresponding waveform diagram;

FIGS. 5A and 5B illustrate side view cross-sections of additionalembodiment variable flow transducer;

FIGS. 6A, 6B, and 6C illustrate side view cross-sections of embodimentacoustic valves;

FIGS. 7A, 7B, 7C, and 7D illustrate top views of further embodimentvariable flow transducers;

FIGS. 8A and 8B illustrate side view cross-sections of more embodimentvariable flow transducers;

FIGS. 9A, 9B, and 9C illustrate side view cross-sections and a top viewof another embodiment variable flow transducer;

FIGS. 10A, 10B, and 10C illustrate waveform diagrams of embodimentvariable flow transducer operation;

FIG. 11 illustrates an additional waveform diagram of embodimentvariable flow transducer operation; and

FIG. 12 illustrates a flowchart diagram of embodiment method ofoperation for a variable flow transducer.

Corresponding numerals and symbols in the different figures generallyrefer to corresponding parts unless otherwise indicated. The figures aredrawn to clearly illustrate the relevant aspects of the embodiments andare not necessarily drawn to scale.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

The making and using of various embodiments are discussed in detailbelow. It should be appreciated, however, that the various embodimentsdescribed herein are applicable in a wide variety of specific contexts.The specific embodiments discussed are merely illustrative of specificways to make and use various embodiments, and should not be construed ina limited scope.

Description is made with respect to various embodiments in a specificcontext, namely acoustic transducers, and more particularly, MEMSmicrospeakers. Some of the various embodiments described herein includeMEMS microspeakers, acoustic transducer systems, variable volume flowtransducers, and variable volume flow MEMS microspeakers. In otherembodiments, aspects may also be applied to other applications involvingany type of transducer domain according to any fashion as known in theart.

Speakers are transducers that transduce electrical signals into acousticsignals. The acoustic signal is produced by the speaker structuregenerating pressure oscillations at a frequency. For example, theaudible range of humans is about 20 Hz to 20 kHz, with some humans ableto hear less than this range and some humans able to hear beyond thisrange. Thus, a speaker operating in order to produce audible acousticsignals transduces electrical signals into sound pressure oscillationswith frequencies between 20 Hz and 20 kHz. A constant frequency signalis conveyed as a simple tone, similar to a note on a piano. Speech andother typical sounds such as, e.g., music, are composed of numerousacoustic signals with numerous frequencies at the same time.

Microspeakers operate according to the same principles as speakers, butare produced using micromachining or microfabrication techniques. Thus,audible microspeakers include small structures that are excited byelectrical signals in order to generate pressure oscillations in theaudible frequency range.

According to various embodiments, a speaker, or microspeaker, isconfigured to generate audible acoustic signals by oscillating atfrequencies above the audible frequency range. In such embodiments, thespeaker is configured to generate pressure oscillations at a frequencyabove the audible range and modify the volume flow of the pressureoscillations according to a lower frequency in the audible frequencyrange. In such embodiments, the human auditory system will recognize theenvelope of the pressure oscillations and act like a low pass filer. Inadditional embodiments, the speaker may be configured to generatepressure oscillations at a frequency above the audible range and modifythe volume flow of the pressure oscillations according to a lowerfrequency still outside the audible frequency range in order to operateas an ultrasound transducer.

In various embodiments, the speaker is referred to as a variable flowtransducer. The frequency of the variable flow transducer may maintainoperation outside the audible frequency range, while the volume flowalters the positive and negative sound pressures of the oscillationsaccording to other frequencies that are inside the audible frequencyrange. In such embodiments, the variable flow transducer may include adeflectable membrane with multiple valve structures that are configuredto adjust the acoustic impedance and alter the volume flow as thedeflectable membrane oscillates above the audible frequency range.Various embodiments are further described herein below.

FIG. 1 illustrates a system block diagram of an embodiment variable flowtransducer 100 including microspeaker 102, application specificintegrated circuit (ASIC) 104, and audio processor 106. According tovarious embodiments, microspeaker 102 generates acoustic signal 108,which includes pressure oscillations at a frequency above the audiblelimit, e.g., 20 kHz, with adjustments of the positive and negative soundpressures during the oscillations. The positive and negative soundpressures may be adjusted by using embodiment valves to adjust theacoustic impedance of a membrane in microspeaker 102. By adjusting thevolume flow through control of the positive and negative soundpressures, low frequency sound pressure signals in the audible range maybe generated from the membrane that oscillates at a frequency above theaudible limit. Thus, microspeaker 102 generates acoustic signal 108including an audible acoustic signal formed from an inaudible acousticsignal. In various embodiments, the pressure oscillations of acousticsignal 108 have a frequency that is at least twice the limit of thehuman auditory range, e.g., 40 kHz, in order to fulfill theNyquist-Shannon sampling theorem.

In various embodiments, microspeaker 102 includes a deflectable membranewith valves. Various example embodiment structures are described furtherherein below. Microspeaker 102 is driven by drive signals provided fromASIC 104. ASIC 104 may generate analog drive signals based on a digitalinput control signal. In some embodiments, ASIC 104 and microspeaker 102are attached to a same circuit board. In other embodiments, ASIC 104 andmicrospeaker 102 are formed on a same semiconductor die. ASIC 104 mayinclude biasing and supply circuits, an analog drive circuit, and adigital to analog converter (DAC). In further embodiments, microspeaker102 may include a microphone, for example, and ASIC 104 may also includereadout electronics such as an amplifier or analog to digital converter(ADC).

In some embodiments, the DAC in ASIC 104 receives a digital controlsignal at an input supplied by audio processor 106. The digital controlsignal is a digital representation of the acoustic signal thatmicrospeaker 102 produces. In various embodiments, audio processor 106may be a dedicated audio processor, a general system processor, such asa central processing unit (CPU), a microprocessor, or a fieldprogrammable gate array (FPGA). In alternative embodiments, audioprocessor 106 may be formed of discrete logic blocks or othercomponents. In various embodiments, audio processor 106 generates thedigital representation of acoustic signal 108 and provides the digitalrepresentation of acoustic signal 108. In other embodiments, audioprocessor 106 provides the digital representation of only the audibleportion of acoustic signal 108 and ASIC 104 generates the driving signalfor acoustic signal 108 with the higher inaudible frequency oscillationsand the audible frequency oscillations based on variations in volumeflow.

According to various additional embodiments, microspeaker 102 may alsogenerate acoustic signal 108, which includes pressure oscillations at afrequency above the audible limit, e.g., 20 kHz, with volume flowadjustments of the sound pressure oscillations that are adjusted atfrequencies that are also above the audible range. For example,microspeaker 102 may operate as an ultrasound transducer for ultrasoundimaging or for ultrasound near field detection. In such embodiments,microspeaker 102 operates with a higher frequency as a carrier signalthat has positive and negative sound pressures adjusted according to alower frequency of the generated target signal, such as an ultrasoundsignal for example.

FIGS. 2a and 2b illustrate waveform diagrams of illustrative acousticsignals. FIG. 2a shows acoustic signal A_(SIG) that may be produced by aspeaker, for example. Acoustic signal A_(SIG) has amplitude A_(amp) andfrequency A_(freq), i.e., period A_(T)=1÷A_(freq). Acoustic signalA_(SIG) may illustrate a sound wave produced by a speaker. Duringoperation, the sound wave has frequency A_(freq) that is within theaudible frequency range for a human, e.g., between about 20 Hz and 20kHz. FIG. 2a illustrates amplitude A_(amp) for acoustic signal A_(SIG)at an unspecified level. For a MEMS microspeaker, generating a largesound pressure level (SPL) may present challenges due to the small sizeof the membrane, especially at low frequency. For example, a MEMSmicrospeaker may include a decrease of 40 dB in SPL per decade asfrequency decreases through the audible frequency range. Thus, it may bechallenging to generate higher SPLs at frequencies below, for example,1-10 kHz without increasing the size of the pumping structure, forexample.

FIG. 2b shows modulated acoustic signal MA_(SIG) that may be produced byan embodiment variable flow transducer, such as a MEMS microspeaker.According to various embodiments, modulated acoustic signal MA_(SIG) hasamplitude MA_(amp) and frequency MA_(freq), i.e., periodMA_(T)=1÷MA_(freq), and is formed of carrier signal C_(SIG), which hasvariable amplitude C_(amp) and frequency C_(freq), i.e., periodC_(T)=1÷C_(freq). As shown, frequency C_(freq) is much higher thanfrequency MA_(freq). Specifically, frequency C_(freq) is above theaudible frequency range of a human, i.e., above 20 kHz, and frequencyMA_(freq) is within the audible frequency range of a human, i.e.,between about 20 Hz and 20 kHz. In such embodiments, amplitude C_(amp)is adjusted in order to form the rising and falling wave form of pumpingacoustic signal PA_(SIG). Further, negative or positive sound pressuresare removed or reduced for carrier signal C_(SIG) in order to form therising and falling wave form of modulated acoustic signal MA_(SIG). Theoscillations of a deflectable membrane generally include symmetricvolume flow that includes equal positive and negative pressure. Invarious embodiments, carrier signal C_(SIG) includes only one type ofsound pressure, e.g., positive sound pressure, for the first half-wave(MA_(T)/2) of modulated acoustic signal MA_(SIG) and only a second typeof sound pressure, e.g., negative sound pressure, for the secondhale-wave (MA_(T)/2) of modulated acoustic signal MA_(SIG). In suchembodiments, carrier signal C_(SIG) shapes the positive sound pressurefirst half-wave of modulated acoustic signal MA_(SIG) by removing (orreducing) the negative sound pressure components and the negative soundpressure second half-wave of modulated acoustic signal MA_(SIG) byremoving (or reducing) the positive sound pressure components. Thevariation of amplitude C_(amp) and direction of carrier signal C_(SIG),through the reducing or removing of positive or negative soundpressures, is performed at a specific frequency in order to formmodulated acoustic signal MA_(SIG) with frequency MA_(freq), which is inthe audible range, e.g., 20 Hz to 20 kHz. According to variousembodiments, variable flow transducers adjust the acoustic impedance ofa deflectable membrane in order to reduce or remove negative or positivesound pressures.

In particular embodiments, amplitude MA_(amp) of modulated acousticsignal MA_(SIG) may be larger than a traditional microspeaker thatoscillates at an audible frequency. In specific embodiments, theoscillation of the pumping speaker remains at a higher frequency suchthat the SPL of modulated acoustic signal MA_(SIG) does not decreasemuch or at all when frequency MA_(freq) is below about 1-10 kHz andabove about 10 Hz, for example. According to various embodiments, theproduced sound or pressure pulses of modulated acoustic signal MA_(SIG)are equal to, or approximately equal to, the second derivative of thedeflectable membrane position, which is the acceleration of thedeflectable membrane. Thus, in various embodiments, the control of thepumping action, such as the control of the positive and negative soundpressures, may be based on the acceleration of the deflectable membrane.

In various embodiments, frequency C_(freq) may be held constant asamplitude C_(amp) and direction of carrier signal C_(SIG) are varied. Inspecific embodiments, frequency C_(freq) may be matched to the resonantfrequency of the speaker or microspeaker in order to produce greateroscillations of the deflectable membrane. In other embodiments,frequency C_(freq) may be variable. In particular examples, frequencyC_(freq) is between 40 kHz and 10 MHz. In more specific embodiments,frequency C_(freq) is between 100 kHz and 300 kHz. In such variousembodiments, frequency MA_(freq) is below 20 kHz. Specifically,frequency MA_(freq) is in the audible frequency range of humans, i.e.,between 20 Hz and 20 kHz, where this range may be expanded for somehumans and narrowed for others. In alternative embodiments, frequencyMA_(freq) may be above 20 kHz. In such embodiments, modulated acousticsignal MA_(SIG) may be, instead of an acoustic signal, an ultrasoundsignal used in an ultrasound transducer for ultrasound imaging or nearfield detection.

According to various embodiments, variable flow transducers, such asMEMS microspeakers, are operated as described in reference to FIG. 2b byusing a carrier signal above the audible frequency range to form amodulated acoustic signal within the audible frequency range. Variousembodiment variable flow transducers are described herein below in orderto illustrate some of the specific applications including capacitiveplate structures and other pumping structures. Such embodiment variablevolume flow transducers adjust the acoustic impedance of the deflectablemembrane in order to reduce or remove negative or positive soundpressures.

Referring back to FIG. 1 in view of FIGS. 2a and 2b , ASIC 104 invariable flow transducer 100 is configured to determine the resonantfrequency of microspeaker 102 in some embodiments. In such embodiments,ASIC 104 may excite microspeaker 102 at a plurality of frequencies andmeasure the response for each frequency. Based on the measured response,ASIC 104 determines the resonant frequency of microspeaker 102. In suchembodiments, ASIC 104 may set frequency C_(freq) for carrier signalC_(SIG) to the determined resonant frequency. In other alternativeembodiments, ASIC 104 may control elements of microspeaker 102 in orderto adjust the resonant frequency to match frequency C_(freq) for carriersignal C_(SIG). In one embodiment, controlling the elements includesadjusting mechanical components of microspeaker 102. In an alternativeembodiment, controlling the elements includes adjusting active orpassive electrical components of microspeaker 102.

FIGS. 3A, 3B, and 3C illustrate side view cross-sections of anembodiment variable flow transducer 110. According to variousembodiments, variable flow transducer 110 adjusts the acoustic impedanceduring oscillations in order to regulate the generation of positive andnegative sound pressures. In various embodiments, variable flowtransducer 110 includes membrane 112, acoustic valves 114, and actuatingstructures 116. In such embodiments, actuating structures 116 mayinclude a piezoelectric layer or layers configured to generate a forceon membrane 112 based on an applied voltage. Actuating structures 116are formed on a surface of membrane 112 in actuation area 122 a.Actuating structures 116 may be formed on the top surface of membrane112 in some embodiments, as illustrated, or may be formed on the bottomsurface of membrane 112 in other embodiments. In further embodiments,actuating structures 116 may be formed on the top and bottom surfaces ofmembrane 112. In such embodiments, the driving force is inversed betweentop and bottom actuating structures 116.

In various embodiments, an electrical drive signal, such as a controlvoltage, is provided to actuating structures 116 in order to excitemembrane 112 to oscillate at a first frequency above the audible range,i.e., above 20 kHz. For example, in some embodiments, membrane 112 isexcited to oscillate at a resonant frequency, which may range from 75kHz to 200 kHz. In such embodiments, the first frequency may correspondto frequency C_(freq) for carrier signal C_(SIG), as describedhereinabove in reference to FIG. 2B. Thus, membrane 112 oscillates withupward and downward movements as shown in FIGS. 3B and 3C. In variousembodiments, acoustic valves 114 are closed for movement in a firstdirection, such as displayed in FIG. 3B during a positive acceleration,and open during the negative acceleration as it occurs during braking ofthe membrane. FIG. 3C shows the second direction, where positiveacceleration occurs in this direction and negative acceleration occursin the inverse direction.

In various embodiments, membrane 112 has a first acoustic impedance whenacoustic valves 114 are closed and a second acoustic impedance whenacoustic valves 114 are open. The first impedance is much greater thanthe second impedance. In such embodiments, when the acoustic impedanceis higher, i.e., when acoustic valves 114 are closed, the sound pressuregenerated by oscillations of membrane 112 are at a normal or largelevel. Conversely, when the acoustic impedance is lower, i.e., whenacoustic valves 114 are open, the sound pressure generated byoscillations of membrane 112 are at a lower or reduced level. Thus, invarious embodiments, variable flow transducer 110 is configured toadjust the acoustic impedance of membrane 112 by opening and closingacoustic valves 114 and generate normal or large sound pressure levelsin a positive acceleration and lower or reduced sound pressure levels ina negative acceleration.

In various embodiments, the acoustic impedance of membrane 112 may beadjusted to be acoustically transparent for a certain percentage of themembrane area when acoustic valves 114 are open. For example, in someembodiments, the quality and the area of acoustic valves 114 causemembrane 112 to be 90% acoustically transparent in a particularembodiment. In another particular embodiment, the quality and the areaof acoustic valves 114 cause membrane 112 to be 50% acousticallytransparent. In other embodiments, the acoustic transparency of membrane112 may range from 30% to 95%.

As described hereinabove in reference to FIG. 2B, by adjusting theacoustic impedance of membrane 112 to be large during positiveacceleration in upward movements and reduced during braking or negativeacceleration for upward movements (as shown in FIG. 3B), variable flowtransducer 110 may remove or reduce negative or positive sound pressuresand form a first half-wave of an acoustic signal having a secondfrequency that is within the audible range. In such embodiments, thesecond frequency may correspond to frequency MA_(freq) for modulatedacoustic signal MA_(SIG), as described hereinabove in reference to FIG.2B. Similarly, by adjusting the acoustic impedance of membrane 112 to belarge during downward movements for positive accelerations and reducedduring downward movements for negative acceleration (as shown in FIG.3C), variable flow transducer 110 may remove or reduce negative soundpressures and form a second half-wave of the acoustic signal. Thus, bymodulating the acoustic impedance to control the generate soundpressures, membrane 112 may oscillate at the first frequency, that isoutside the audible range, and generate an acoustic signal at the secondfrequency, that is within the audible range. In such variousembodiments, similar efforts or techniques referred to as digital soundreconstruction may be implemented.

In various embodiments, acoustic valves 114 include piezoelectricmaterials that open and close acoustic valves 114 based on electricalcontrol signals. Acoustic valves 114 are formed throughout ventilationarea 122 b of membrane 112. In various embodiments, membrane 112 isformed of structural layer 118 and isolation layer 120. In someembodiments, structural layer 118 is a conductive layer, such as asemiconductor or metal, and isolation layer 120 is an electricallyinsulating layer, such as an oxide layer, a nitride layer, or anoxynitride layer. In other embodiments, structural layer 118 andisolation layer 120 may be combined into a single conductive orelectrically insulating layer. As shown, membrane 112 may be anchored toa support structure at a periphery. Further structure details of variousembodiments are described hereinafter in reference to the other Figures.In other embodiments, acoustic valves 114 or membrane 112 may beactuated electrostatically, instead of piezoelectrically as shown.

FIGS. 4A, 4B, and 4C illustrate an embodiment model variable flowtransducer and a corresponding waveform diagram. Specifically, FIG. 4Adepicts annotated variable flow transducer 130, FIG. 4B depicts pistonmodel 132, and FIG. 4C depicts membrane displacement waveform 134 andmembrane acceleration waveform 136. According to various embodiments,when acoustic valves 114 are closed, membrane 112 has a high acousticimpedance, as illustrated by closed valve portion 138 of annotatedvariable flow transducer 130 and piston model 132. Conversely, whenacoustic valves 114 are open, membrane 112 has a low acoustic impedance,as illustrated by open valve portion 142 of annotated variable flowtransducer 130 and piston model 132. Transition between acoustic highimpedance and acoustic low impedance is depicted by transition portion140. In such embodiments, oscillations of membrane 112 may be modeledwith equal displacement of the entire membrane according to piston model132. When membrane 112 has a low acoustic impedance, the acousticmedium, such as air, is able to easily pass from one side of membrane112 to the other. When membrane 112 has a high acoustic impedance, theacoustic medium, such as air, is unable to easily pass from one side ofmembrane 112 to the other.

According to various embodiments, transitioning from closed valveportion 138 to open valve portion 142 may be based on the accelerationof membrane 112. As illustrated by membrane displacement waveform 134and membrane acceleration waveform 136, when acceleration of membrane112 has a positive value, acoustic valves 114 are closed, and whenacceleration of membrane 112 has a negative value, acoustic valves 114are open. In such embodiments, the positive and negative sign of theacceleration may be switched based on the half-wave of the acousticsignal, positive or negative half-wave (see FIG. 2B), being generated.In various embodiments, the acoustic impedance may be adjusted based onthe displacement or acceleration of membrane 112 in order to selectivelygenerate positive or negative sound pressure waves for forming audibleacoustic signals.

Further embodiment variable flow transducers are described hereinafteras illustrative embodiments.

FIGS. 5A and 5B illustrate side view cross-sections of additionalembodiment variable flow transducer 150 and embodiment variable flowtransducer 151. According to various embodiments, variable flowtransducer 150 includes substrate 152, membrane 154, top backplate 156or bottom backplate 158, and acoustic valves 160. Acoustic valves 160are shown generically as dashed structures and may be implemented aspiezoelectric or electrostatic controllable valves. Example embodimentacoustic valves are described further hereinafter in reference to FIGS.6A, 6B, and 6C.

In various embodiments, membrane 154 is a deflectable membrane that isactuated electrostatically by applying a voltage difference betweenmembrane 154 and top backplate 156 or between membrane 154 and bottombackplate 158. In some embodiments, variable flow transducer 150 is adual backplate microspeaker that includes both top backplate 156 andbottom backplate 158. In other embodiments, variable flow transducer 150is a single backplate microspeaker that includes either top backplate156 or bottom backplate 158. In various embodiments, top backplate 156and bottom backplate 158 include perforations 157 that allow fluidictransport from one side of top backplate 156 or bottom backplate 158 tothe other side. In such embodiments, the fluidic transport allowsacoustic signals to pass through top backplate 156 and bottom backplate158, which provide a low acoustic impedance.

In various embodiments, membrane 154 is electrostatically driven tooscillate at a frequency above the audible range. In specificembodiments, membrane 154 oscillates with a frequency ranging from 40kHz to 300 kHz. During oscillations, acoustic valves 160 are controlledto regulate generation of positive or negative sound pressures fromoscillations of membrane 154 and form modulated acoustic signals thathave frequencies within the audible range, as described hereinabove inreference to FIGS. 2A, 2B, 3A, 3B, 3C, 4A, 4B, and 4C.

In some embodiments, bypass route 166, bypass structure 162, andacoustic valves 160 in bypass structure 162 are included surroundingmembrane 154. In other embodiments, bypass route 166, bypass structure162, and acoustic valves 160 in bypass structure 162 are omitted. Insome embodiments including bypass route 166, acoustic valves 160 onmembrane 154 may be omitted. In other embodiments including bypass route166, acoustic valves 160 on membrane 154 are included.

In various embodiments, substrate 152 is formed of a semiconductormaterial. For example, substrate 152 may be silicon, such aspolysilicon, gallium-arsenide (GaAs), indium phosphide (InP), or carbonin particular embodiments. In other embodiments, substrate 152 is formedof a dielectric material such as a glass. In still further embodiments,substrate is formed of a polymer, such as hexamethyldisilazane (HMDS)for example. In other alternative embodiments, substrate 152 is formedof a ceramic material. In various embodiments, membrane 154 is formed ofa semiconductor or a metal, such as polysilicon, gold, aluminum, copper,or platinum. In other embodiments, membrane 154 formed of anon-conductive layer and a conductive layer. In various embodiments, topbackplate 156 and bottom backplate 158 are formed of a semiconductor ora metal, such as polysilicon, gold, aluminum, copper, or platinum. Infurther embodiments, top backplate 156 and bottom backplate 158 areformed of multiple layers including conductive layers and non-conductiveor electrically insulating layer. For example, in a particularembodiment, top backplate 156 and bottom backplate 158 are formed ofpolysilicon and silicon nitride. Substrate 152 includes cavity 164,which may pass through the entirety of substrate 152, such as through awafer including substrate 152.

According to various embodiments, variable flow transducer 151 includessubstrate 152, membrane 168, and acoustic valves 160. In suchembodiments, membrane 168 is a deflectable membrane that is actuatedpiezoelectrically by applying a voltage signal to piezoelectric layer170. By applying a voltage signal to piezoelectric layer 170, adeformation is generated in piezoelectric layer 170 that generates aforce on membrane 168. The excitation of membrane 168 is performed at ahigher frequency above the audible range and acoustic valves 160 arecontrolled to form modulated acoustic signals that have frequencieswithin the audible range, as described hereinabove in reference tovariable flow transducer 150 in FIG. 5A.

In various embodiments, membrane 168 includes structural layer 172,isolation layer 174, and piezoelectric layer 170. In some embodiments,structural layer 172 is a conductive layer, such as a semiconductorlayer or a metal layer. Isolation layer 174 may be an electricallyinsulating layer, such as an oxide layer, a nitride layer, or anoxynitride layer. In various embodiments, piezoelectric layer 170includes piezoelectric ceramics or piezoelectric crystals. In particularembodiments, piezoelectric layer 170 includes lead zirconate titanate(PZT) or barium titanate (BaTiO₃). In other particular embodiments,piezoelectric layer 170 includes zinc oxide (ZnO), aluminum nitride(AlN), or polyvinylidene fluoride (PVDF).

According to various embodiments, variable flow transducer 150 andvariable flow transducer 151 are illustrated in FIGS. 5A and 5B incross-section and may include any membrane shape when viewed from above.Specifically, membrane 154 and membrane 168 may be round, includingcircular or oval shapes, or rectangular in particular embodiments. Insome embodiments, bypass route 166 is omitted and substrate 152 extendsto and surrounds membrane 154 or membrane 168. In other embodiments,bypass route 166 is included and substrate 152 includes a portionsurrounding and supporting membrane 154 or membrane 168 that isconnected to the main portion of substrate 152. In such embodiments,portions of the perimeter of membrane 154 or membrane 168 include bypassroute 166 and other portions of the perimeter of membrane 154 ormembrane 168 include solid portions of substrate 152. Various embodimentvariable flow transducers are described hereinafter in reference to topviews illustrated in FIGS. 7A, 7B, 7C, and 7D.

FIGS. 6A, 6B, and 6C illustrate side view cross-sections of embodimentacoustic valves 180, 181, and 182. According to various embodiments,acoustic valve 180, acoustic valve 181, or acoustic valve 182 may beused to implement any of the acoustic valves described herein, such asacoustic valve 114 or acoustic valve 160 as described hereinabove.

According to various embodiments, acoustic valve 180 includes structurallayer 184, isolation layer 186, acoustic flap 188, and piezoelectriclayer 190. In various embodiments, piezoelectric layer 190 may includeany of the materials described hereinabove in reference to piezoelectriclayer 170. Piezoelectric layer 190 is disposed on acoustic flap 188. Invarious embodiments, acoustic flap 188 has mechanical elasticity. Inparticular embodiments, acoustic flap 188 is single crystal silicon orpolysilicon. In various further embodiments, acoustic flap 188 may beany type of electrically insulating material with suitable mechanicalproperties for actuation. In still further embodiments, acoustic flap188 may include any type of electrically conductive material with aninsulating layer. In specific embodiments, acoustic flap 188 is graphenewith an insulating layer. In various embodiments, piezoelectric layer190 extends over only part of the top surface of acoustic flap 188, asshown. In alternative embodiments, piezoelectric layer 190 extends overthe entire top surface of acoustic flap 188 (not shown). In alternativeembodiments, piezoelectric layer 190 can be shaped in various ways toachieve different transient valve characteristics due to structural ormechanical interactions. For example, piezoelectric layer 190 may beshaped with a solid region, a comb region, a circular region, or anothershape in order to adjust the transient valve characteristics.

In various embodiments, acoustic flap 188 seals opening 185 instructural layer 184 and isolation layer 186. When an electrical drivesignal, such as a control voltage, is applied to piezoelectric layer190, piezoelectric layer 190 begins to deform, causing a force onacoustic flap 188. The force on acoustic flap 188 moves acoustic flap188 to open and allow fluid transport through opening 185. In someembodiments, a first control voltage is applied to piezoelectric layer190 to close acoustic flap 188 and seal opening 185, and a secondcontrol voltage is applied to piezoelectric layer 190 to open acousticflap 188 and open opening 185.

In various embodiments, isolation layer 186 is an electricallyinsulating material. In some embodiments, isolation layer 186 is anoxide, nitride, or oxynitride. In particular embodiments, isolationlayer 186 is silicon nitride (SiN) or silicon oxide (SiO₂). According tovarious embodiments, structural layer 184 is an electrically conductiveor semiconductive material. In some embodiments, structural layer 184 isa crystalline or amorous semiconductor element or compound. Inparticular embodiments, structural layer 184 is polysilicon. In otherembodiments, structural layer 184 is a metal. In particular embodiments,structural layer 184 is aluminum, platinum, gold, or copper. In variousembodiments, structural layer 184 may be a portion of a deflectablemembrane, such as described herein in reference to the other figures.

According to various embodiments, acoustic valve 181 includes structurallayer 184, isolation layer 186, acoustic flap 192, and piezoelectriclayer 194. In such embodiments, acoustic flap 192 is a portion ofstructural layer 184. Piezoelectric layer 194 may include any of thematerials described hereinabove in reference to piezoelectric layer 190in FIG. 6A. Further, piezoelectric layer 190 may extend over only aportion of the top surface of acoustic flap 192, as shown. Inalternative embodiments, piezoelectric layer 190 extends over the entiretop surface of acoustic flap 192 (not shown).

According to various embodiments, acoustic valve 182 includes structurallayer 184, isolation layer 186, structural support 196, andelectrostatic seal layer 198. In such embodiments, a control voltage isapplied to electrostatic seal layer 198 in order to generate anelectrostatic force that closes electrostatic seal layer 198 and sealsopening 185. In various embodiments, electrostatic seal layer 198 is aconductive or semiconductive material. In various particularembodiments, electrostatic seal layer 198 is polysilicon, gold,aluminum, cooper, or platinum. Structural support 196 is formed of anelectrically insulating structural material. In some embodiments,structural support 196 is formed oxide, such as tetraethyl orthosilicate(TEOS) oxide.

In various embodiments, in order to generate an electrostatic force onelectrostatic seal layer 198, a voltage difference is applied betweenelectrostatic seal layer 198 and structural layer 184. When the voltagedifference is applied, electrostatic seal layer 198 seals opening 185and when no voltage difference is applied, electrostatic seal layer 198moves away from opening 185 and allows fluid transport through opening185.

FIGS. 7A, 7B, 7C, and 7D illustrate top views of further embodimentvariable flow transducers 200 a, 200 b, 200 c, and 200 d. FIG. 7Aillustrates variable flow transducer 200 a including support structure202, membrane 204, and acoustic valves 206. According to variousembodiments, membrane 204 is driven to oscillate above a higher firstfrequency and acoustic valves 206 are controlled to open and close inorder to shape the positive and negative sound pressures that formacoustic signals with frequencies below a lower second frequency. Insome embodiments, membrane 204 may oscillate with a frequency rangingfrom 40 kHz to 300 kHz and acoustic valves 206 may be opened and closedto form acoustic signals with frequencies ranging from 20 Hz to 20 kHz.

In such embodiments, acoustic valves 206 may be implemented as describedhereinabove in reference to acoustic valves 114, 160, 180, 181, or 182in reference to the other figures. In particular embodiments, acousticvalves 206 correspond to acoustic valve 180 or acoustic valve 181 asdescribed hereinabove in reference to FIGS. 6A and 6B, respectively. Inspecific embodiments, acoustic valves 206 include acoustic flaps 208 andpiezoelectric actuation layers 210 formed on a top surface of theacoustic flap 208.

In various embodiments, support structure 202 may be a substrate, suchas described hereinabove in reference to substrate 152 in FIGS. 5A and5B. In other embodiments, support structure 202 may be an oxide, such asa TEOS oxide, or a polymer. In such embodiments, support structure 202may be formed on a substrate. Membrane 204 may include any of thestructures and materials as described hereinabove in reference tomembrane 154 or membrane 168 in FIGS. 5A and 5B, respectively. Invarious embodiments a cavity is formed in the substrate below membrane204.

FIG. 7B illustrates variable flow transducer 200 b including supportstructure 202, membrane 204, and acoustic valves 212. According tovarious embodiments, variable flow transducer 200 b is similar tovariable flow transducer 200 a, with the exception that acoustic valves206, which are piezoelectrically actuated, are replaced by acousticvalves 212, which are electrostatically actuated. In such embodiments,acoustic valves 212 correspond to acoustic valve 182 as describedhereinabove in reference to FIG. 6C. Acoustic valves 212 includeelectrostatic seal layer 214.

FIG. 7C illustrates variable flow transducer 200 c including supportstructure 202, membrane 204, and acoustic valves 216. According tovarious embodiments, acoustic valves 216 are formed in support structure202 around membrane 204. In such embodiments, acoustic valves 216correspond to bypass route 166, bypass structure 162, and acousticvalves 160 in bypass structure 162 as described hereinabove in referenceto FIGS. 5A and 5B.

In particular embodiments, acoustic valves 216 may be implemented asdescribed hereinabove in reference to acoustic valves 114, 160, 180,181, or 182 in reference to the other figures. In such embodiments,acoustic valves 216 may include multiple separate acoustic valves, suchas with square acoustic flaps or continuous curved acoustic valvessurrounding the perimeter of membrane 204. Acoustic valves 216 may beelectrostatically or piezoelectrically actuated in differentembodiments. In other embodiments, membrane 204 may also includeacoustic valves (not shown), such as described hereinabove in referenceto variable flow transducer 200 a and variable flow transducer 200 b inFIGS. 7A and 7B, respectively.

FIG. 7D illustrates variable flow transducer 200 d including supportstructure 202, membrane 204, and acoustic flaps 220. According tovarious embodiments, acoustic valves 218 are formed in membrane 204.Membrane slits 222 in membrane 204 allow acoustic flaps 220 to deflectseparately from membrane 204. In such embodiments, piezoelectricactuation layers 224 are formed on a top surface of membrane 204 andcause acoustic flaps 220 to deflect when a control signal, such as anactuation voltage is applied to piezoelectric actuation layers 224. Invarious embodiments, acoustic valves 218 correspond to acoustic valve181 as described hereinabove in reference to FIG. 6B. In otherembodiments, variable flow transducer 200 d and acoustic valves 218 maybe modified to correspond to acoustic valve 180 as described hereinabovein reference to FIG. 6A.

In various embodiments, variable flow transducers 200 a, 200 b, 200 c,and 200 d include circular membranes, as shown. In other embodiments,variable flow transducers 200 a, 200 b, 200 c, and 200 d may includeoval or rectangular membranes (not shown). In still further embodiments,variable flow transducers 200 a, 200 b, 200 c, and 200 d may include anyshape of membrane, such as hexagonal or octagonal, for example.

FIGS. 8A and 8B illustrate side view cross-sections of more embodimentvariable flow transducers 111 a and 111 b. Variable flow transducers 111a and 111 b each include membrane 112, acoustic valves 114, andactuating structures 116 as described hereinabove in reference tovariable flow transducer 110 in FIGS. 3A, 3B, and 3C. According tovarious embodiments, acoustic valves 114 are included in variousdifferent numbers and configurations. Specifically, variable flowtransducer 111 a includes acoustic valves 114 are arranged in centralregion 123 a, as shown in FIG. 8A. In such embodiments, peripheralregion 123 b is solid. In various embodiments, the deflection ofmembrane 112 is largest near the center and smallest near the anchor.

In other embodiments, variable flow transducer 111 b includes acousticvalves 114 arranged in peripheral region 123 b, as shown in FIG. 8B. Insuch embodiments, central region 123 a is solid. In such embodiments,actuating structures 116 may be formed on the top surface of membrane112 at the edge of central region 123 a. According to variousembodiments, any number and arrangement of acoustic valves may bearranged in any portion of membrane. Further, FIGS. 7A and 7B illustrateonly a single row of acoustic valves arranged in a circle on membrane204, but various other embodiments may include two, three, or moreacoustic valves arranged in concentric circles on a membrane, such asillustrated for variable flow transducer 111 a and variable flowtransducer 111 b in FIGS. 8A and 8B, respectively. Those having skill inthe art will readily appreciate various modifications of the number andconfiguration of embodiment acoustic valves for embodiment variable flowtransducers. Such modifications are well within the scope of theembodiments described herein.

FIGS. 9A, 9B, and 9C illustrate side view cross-sections and a top viewof another embodiment variable flow transducer 250 including bottommembrane 252 and top membrane 254. According to various embodiments,bottom membrane 252 includes acoustic vents 256 and top membrane 254includes acoustic vents 258. Acoustic vents 256 and acoustic vents 258are offset so that the vents do not overlap. In such embodiments, when avoltage difference is applied between bottom membrane 252 and topmembrane 254, an electrostatic force attracts bottom membrane 252 andtop membrane 254 together and seals acoustic vents 256 and acousticvents 258, as shown in FIG. 9B. When no voltage difference or a smallvoltage difference is applied between bottom membrane 252 and topmembrane 254, the membranes stay separated and acoustic vents 256 andacoustic vents 258 are open, as shown in FIG. 9A.

According to various embodiments, when acoustic vents 256 and acousticvents 258 are sealed, bottom membrane 252 and top membrane 254 areacoustically solid, i.e., acoustically visible or acoustically opaque.When acoustic vents 256 and acoustic vents 258 are open, bottom membrane252 and top membrane 254 are acoustically transparent.

In various embodiments, bottom membrane 252 and top membrane 254 aredriven to oscillate above a higher first frequency and acoustic vents256 and acoustic vents 258 are controlled to open and seal in order toshape the positive and negative sound pressures that form acousticsignals with frequencies below a lower second frequency. In someembodiments, bottom membrane 252 and top membrane 254 may oscillate witha frequency ranging from 40 kHz to 300 kHz and acoustic vents 256 andacoustic vents 258 may be opened and sealed to form acoustic signalswith frequencies ranging from 20 Hz to 20 kHz.

An embodiment arrangement of acoustic vents 256 and acoustic vents 258is shown in FIG. 9C. In various embodiments, acoustic vents 256 andacoustic vents 258 may be arranged in any type of random arrangement ornonrandom pattern.

According to various embodiments, bottom membrane 252 and top membrane254 are driven to oscillate either piezoelectrically orelectrostatically. Specifically, bottom membrane 252 and top membrane254 may be arranged with top or bottom perforated backplates orpiezoelectric actuation layers, such as described hereinabove inreference to variable flow transducer 150 and variable flow transducer151 in FIGS. 5A and 5B, respectively. In such embodiments, bottommembrane 252 and top membrane 254 are driven together to oscillate atthe higher frequency above the audible frequency range.

According to alternative embodiments, bottom membrane 252 and topmembrane 254 may be actuated to open and seal acoustic vents 256 andacoustic vents 258 piezoelectrically. In such embodiments, optionalpiezoelectric actuation layers 255 are formed on bottom membrane 252 andtop membrane 254 in order to provide forces to open and seal acousticvents 256 and acoustic vents 258.

FIGS. 10A, 10B, and 10C illustrate waveform diagrams of embodimentvariable flow transducer operation. FIGS. 10A, 10B, and 10C includewaveform diagrams shown on a normalized vertical axis versus time. FIG.10A illustrates membrane displacement waveform 270 and membraneacceleration waveform 272, as similarly described hereinabove inreference to membrane displacement waveform 134 and membraneacceleration waveform 136 in FIG. 4C. According to various embodiments,a membrane is driven, piezoelectrically or electrostatically, tooscillate at a frequency or frequencies above the audible range. Forexample, the membrane may be driven to oscillate at a resonant frequencyof the membrane, such as, e.g., 100 kHz.

In various embodiments, the acoustic impedance of the membrane isadjusted during the oscillations in order to generate a modulatedacoustic signal. In some embodiments, acoustic valves are opened whenthe membrane is decelerating, which may be referred to as braking. FIG.10B illustrates braking waveform 276 and volume flow waveform 274, whichcorrespond to accelerations and decelerations of membrane accelerationwaveform 272 in FIG. 10A.

When the membrane is decelerating, i.e., when braking waveform 276 has avalue of 1, the acoustic valves are open. In such embodiments, themembrane is acoustically transparent, e.g., the acoustic impedance isdecreased, and the volume flow of the acoustic medium, e.g., air, isdecreased as shown by volume flow waveform 274. In some embodiments,during braking period 280, when braking waveform 276 has a value of 1,the volume flow is half, as shown by volume flow waveform 274. In suchembodiments, the membrane is 50% acoustically transparent during brakingperiod 280 when the acoustic valves are open. In other embodiments, themembrane may have other values for acoustic transparency. In variousembodiments, the membrane is between 30% and 95% acousticallytransparent when the acoustic valves are open. In specific embodiments,the membrane is between 50% and 80% acoustically transparent when theacoustic valves are open. In such various embodiments, the volume flowcorresponds to the acoustic transparency. In some embodiments, acoustictransparence may also be referred to as an acoustic short circuit.

When the membrane is accelerating, i.e., when braking waveform 276 has avalue of 0, the acoustic valves are closed. In such embodiments, themembrane is acoustically opaque, e.g., the acoustic impedance isincreased or at a maximum, and the volume flow of the acoustic medium,e.g., air, is increased as shown by volume flow waveform 274. In someembodiments, during accelerating period 278, when braking waveform 276has a value of 0, the volume flow is full, as shown by volume flowwaveform 274.

FIG. 10C illustrates 100% volume flow waveform 282 and 50% volume flowwaveform 284, corresponding to braking waveform 276 and volume flowwaveform 274 in FIG. 10B. The volume flow for a membrane withoutembodiment acoustic valves, as described herein, may be equal forpositive displacements (1,0) and negative displacements (0,−1) as shownby 100% volume flow waveform 282. According to various embodiments, thevolume flow for a membrane with embodiment acoustic valves, as describedherein, may be controlled to have different values for positivedisplacements (1,0) and negative displacements (0,−1) as shown by 50%volume flow waveform 284. In particular embodiments, the membrane is 50%acoustically transparent when the acoustic valves are open, such asduring braking, which produces 50% of the volume flow (for negativevalues of 50% volume flow waveform 284). When the acoustic valves areclosed, such as during accelerating, the membrane is acousticallyopaque, which produces 100% of the volume flow (for positive values of50% volume flow waveform 284).

According to various embodiments, the polarity of the acoustic valvecontrol may be switched in order to shape both positive and negativehalf-waves of an audible acoustic signal. By opening and closing theacoustic valves strategically, positive and negative sound pressurelevels may be shaped from higher frequency oscillations. In variousembodiments, the quality of the acoustic transparency, which may bereferred to as the acoustic impedance or acoustic short circuit, isrelated to the number, size, shape, distribution, and operation of theacoustic valves as described hereinabove in reference to the otherfigures.

FIG. 11 illustrates an additional waveform diagram of embodimentvariable flow transducer operation including high frequency waveform290, high frequency waveform 292, and modulated acoustic waveform 294.According to various embodiments, high frequency waveform 290 and highfrequency waveform 292 are carrier signals having frequencies above theaudible frequency range, such as described hereinabove in reference tocarrier signal C_(SIG) in FIG. 2B. Modulated acoustic waveform 294 is amodulated signal formed from high frequency waveform 290 or highfrequency waveform 292, such as described hereinabove in reference tomodulated acoustic signal MA_(SIG) in FIG. 2B.

According to various embodiments, the quality of the acoustic valves,and the corresponding acoustic pathways or perforations in the membrane,affects the acoustic transparency of the membrane. In particularembodiments, high frequency waveform 290 corresponds to a membrane thatis 50% acoustically visible (50% acoustically transparent) when theacoustic pathways or valves are open and high frequency waveform 292corresponds to a membrane that is 10% acoustically visible (90%acoustically transparent) when the acoustic pathways or valves are open.In such embodiments, the membrane produces full volume flow in thepositive acceleration state and reduced volume flow in the negativeacceleration state due to the acoustic transparency during the firsthalf-wave from 0 to 0.1 ms. Further, the membrane produces full volumeflow in the negative acceleration state and reduced volume flow in thepositive acceleration state due to the acoustic transparency during thesecond half-wave from 0.1 ms to 0.2 ms. The volume flow when theacoustic valves are open is not negligible for the 50% acousticallyvisible membrane, but is dominated by the larger amount of volume flowwhen the acoustic valves are closed. As shown by high frequency waveform290 and high frequency waveform 292, the volume flow when the acousticvalves are open is much greater for the membrane that is 50%acoustically visible than for the membrane that is 10% acousticallyvisible.

According to various embodiments, modulated acoustic waveform 294 isformed or shaped by high frequency waveform 290 or high frequencywaveform 292. In various embodiments, the amplitude of modulatedacoustic waveform 294 may be dependent on the amplitude of highfrequency waveform 290 or high frequency waveform 292 as well as theextent of the acoustic transparency of the membrane when the acousticvalves are open.

FIG. 12 illustrates a flowchart diagram of embodiment method ofoperation 300 for a variable flow transducer. According to variousembodiments, a method of operation 300 is a method of operating a MEMStransducer, where the method includes steps 305, 310, and 315. In suchembodiments, step 305 includes actuating a deflectable membrane tooscillate. The deflectable membrane may oscillate with a frequency orfrequencies above the audible range. For example, in particularembodiments, the deflectable membrane oscillates with a frequency orfrequencies ranging from 40 kHz to 300 kHz.

In various embodiments, step 310 includes controlling a plurality ofcontrollable acoustic paths in the deflectable membrane to provideacoustic low impedance paths between a first volume and a second volumeduring a first mode. The acoustic paths may include controllableacoustic valves as described hereinabove in reference to the otherfigures. Providing the low impedance paths may include opening theacoustic valves in some embodiments. Step 315 includes controlling theplurality of controllable acoustic paths in the deflectable membrane toprovide acoustic high impedance paths between the first volume and thesecond volume during a second mode. Providing the high impedance pathsmay include closing the acoustic valves in some embodiments. In suchembodiments, the high impedance path may include a very large acousticimpedance.

According to an embodiment, a microelectromechanical systems MEMStransducer includes a deflectable membrane attached to a supportstructure, an acoustic valve structure configured to cause thedeflectable membrane to be acoustically transparent in a first mode andacoustically visible in a second mode, and an actuating mechanismcoupled to the deflectable membrane. Other embodiments includecorresponding systems and apparatus, each configured to perform variousembodiment methods.

In various embodiments, the actuating mechanism is configured to exciteoscillations of the deflectable membrane, the oscillations having afrequency above 40 kHz. The MEMS transducer may further include asubstrate, where the support structure is disposed on the substrate. Insome embodiments, the acoustic valve structure includes a plurality ofpiezoelectric valves. In such embodiments, the plurality ofpiezoelectric valves may be formed on the deflectable membrane.

In various embodiments, the acoustic valve structure includes aplurality of electrostatic valves. In such embodiments, the plurality ofelectrostatic valves may be formed on the deflectable membrane. In someembodiments, the actuating mechanism includes a perforated backplateseparated from the deflectable membrane by a separation distance. Inother embodiments, the actuating mechanism includes a piezoelectriclayer formed on the deflectable membrane.

According to an embodiment, a MEMS transducer includes a supportstructure disposed on a substrate, a deflectable membrane supported bythe support structure and separating a first volume from a secondvolume, and an actuation structure coupled to the deflectable membrane.The deflectable membrane includes a plurality of controllable acousticpaths in the deflectable membrane, where each controllable acoustic pathof the plurality of controllable acoustic paths is configured to providean acoustic low impedance path between the first volume and the secondvolume during a first mode, and provide an acoustic high impedance pathbetween the first volume and the second volume during a second mode.Other embodiments include corresponding systems and apparatus, eachconfigured to perform various embodiment methods.

In various embodiments, the actuation structure is configured to excitethe deflectable membrane to oscillate with a frequency above 40 kHz. Insome embodiments, the MEMS transducer further includes a control circuitcoupled to the actuation structure and configured to provide firstcontrol signals to the actuation structure. In such embodiments, thecontrol circuit is may be further configured to provide second controlsignals to the plurality of controllable acoustic paths, and the secondcontrol signals are operable to switch the plurality of controllableacoustic paths between the first mode and the second mode in order toselectively generate positive and negative sound pressures formingaudible acoustic signals with frequencies below 20 kHz while thedeflectable membrane oscillates with the frequency above 40 kHz.

In various embodiments, the plurality of controllable acoustic pathsincludes a plurality of piezoelectric valves formed in the deflectablemembrane. In some embodiments, the plurality of controllable acousticpaths includes a plurality of electrostatic valves formed in thedeflectable membrane.

According to an embodiment, a method of operating a MEMS transducerincludes actuating a deflectable membrane to oscillate, controlling aplurality of controllable acoustic paths in the deflectable membrane toprovide acoustic low impedance paths between a first volume and a secondvolume during a first mode, and controlling the plurality ofcontrollable acoustic paths in the deflectable membrane to provideacoustic high impedance paths between the first volume and the secondvolume during a second mode. Other embodiments include correspondingsystems and apparatus, each configured to perform various embodimentmethods.

In various embodiments, the deflectable membrane is actuated tooscillate with a frequency above 40 kHz. In some embodiments, the methodfurther includes selectively generating positive and negative soundpressures by switching the plurality of controllable acoustic pathsbetween the first mode and the second mode, the positive and negativesound pressures forming audible acoustic signals with frequencies below20 kHz while the deflectable membrane oscillates with the frequencyabove 40 kHz.

In various embodiments, controlling the plurality of controllableacoustic paths in the deflectable membrane to provide acoustic lowimpedance paths may include piezoelectrically opening a plurality ofpiezoelectric acoustic valves, and controlling the plurality ofcontrollable acoustic paths in the deflectable membrane to provideacoustic high impedance paths may include piezoelectrically closing aplurality of piezoelectric acoustic valves. In some embodiments,controlling the plurality of controllable acoustic paths in thedeflectable membrane to provide acoustic low impedance paths includeselectrostatically opening a plurality of electrostatic acoustic valves,and controlling the plurality of controllable acoustic paths in thedeflectable membrane to provide acoustic high impedance paths includeselectrostatically closing a plurality of electrostatic acoustic valves.

According to an embodiment, a MEMS transducer includes a firstdeflectable membrane attached to a support structure and including afirst plurality of perforations, a second deflectable membrane attachedto the support structure and including a second plurality ofperforations, a closing mechanism coupled to the first deflectablemembrane and the second deflectable membrane, and an actuating mechanismconfigured to excite oscillations of the first deflectable membrane andthe second deflectable membrane. The second plurality of perforationsare offset from the first plurality of perforations. The closingmechanism is configured to close an acoustic path through the firstdeflectable membrane and the second deflectable membrane by moving thefirst deflectable membrane and the second deflectable membrane intocontact during a first mode and open the acoustic path by moving thefirst deflectable membrane and the second deflectable membrane out ofcontact during a second mode. In such embodiments, the first pluralityof perforations are sealed to the second deflectable membrane and thesecond plurality of perforations are sealed to the first deflectablemembrane when the acoustic path is closed. Other embodiments includecorresponding systems and apparatus, each configured to perform variousembodiment methods.

In various embodiments, the oscillations of the first deflectablemembrane and the second deflectable membrane have a frequency above 40kHz. In some embodiments, the closing mechanism includes anelectrostatic structure configured to generate an electrostatic forcebetween the first deflectable membrane and the second deflectablemembrane during the first mode. In other embodiments, the closingmechanism includes a piezoelectric structure configured to generate afirst force on the first deflectable membrane and a second force on thesecond deflectable membrane during the first mode, the first force andthe second force configured to move the first deflectable membrane andthe second deflectable membrane into contact.

In various embodiments, the actuating mechanism may include a perforatedbackplate attached to the support structure and configured to generatean electrostatic force between the perforated backplate and the firstdeflectable membrane and the second deflectable membrane. In otherembodiments, the actuating mechanism includes a piezoelectric structureconfigured to generate a first force on the first deflectable membraneand a second force on the second deflectable membrane.

Advantages of various embodiments described herein may include highsound pressure level signals with low frequencies that are formed usinghigher frequency oscillations of a membrane. Other advantages of variousembodiments described herein may include deflectable membranes withcontrollable acoustic impedance. Some advantages of various embodimentsmay include the ability to form positive sound pressures without, orwith reduced, negative sound pressures or the ability to form negativesound pressures without, or with reduced, positive sound pressures.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. A microelectromechanical systems (MEMS)transducer comprising: a first deflectable membrane attached to asupport structure and comprising a first plurality of perforations; asecond deflectable membrane attached to the support structure andcomprising a second plurality of perforations, the second plurality ofperforations offset from the first plurality of perforations; a closingmechanism coupled to the first deflectable membrane and the seconddeflectable membrane, the closing mechanism configured to close anacoustic path through the first deflectable membrane and the seconddeflectable membrane by moving the first deflectable membrane and thesecond deflectable membrane into contact during a first mode, and openthe acoustic path by moving the first deflectable membrane and thesecond deflectable membrane out of contact during a second mode, whereinthe first plurality of perforations are sealed to the second deflectablemembrane and the second plurality of perforations are sealed to thefirst deflectable membrane when the acoustic path is closed; and anactuating mechanism configured to excite oscillations of the firstdeflectable membrane and the second deflectable membrane.
 2. The MEMStransducer of claim 1, wherein the oscillations of the first deflectablemembrane and the second deflectable membrane have a frequency above 40kHz.
 3. The MEMS transducer of claim 1, wherein the closing mechanismcomprises an electrostatic structure configured to generate anelectrostatic force between the first deflectable membrane and thesecond deflectable membrane during the first mode.
 4. The MEMStransducer of claim 1, wherein the closing mechanism comprises apiezoelectric structure configured to generate a first force on thefirst deflectable membrane and a second force on the second deflectablemembrane during the first mode, the first force and the second forceconfigured to move the first deflectable membrane and the seconddeflectable membrane into contact.
 5. The MEMS transducer of claim 1,wherein the actuating mechanism comprises a perforated backplateattached to the support structure and configured to generate anelectrostatic force between the perforated backplate and the firstdeflectable membrane and the second deflectable membrane.
 6. The MEMStransducer of claim 1, wherein the actuating mechanism comprises apiezoelectric structure configured to generate a first force on thefirst deflectable membrane and a second force on the second deflectablemembrane.
 7. A microelectromechanical systems (MEMS) device comprising:a first membrane attached to a support structure and comprising a firstplurality of acoustic vents; a second membrane attached to the supportstructure and comprising a second plurality of acoustic vents, thesecond plurality of acoustic vents offset from the first plurality ofacoustic vents, wherein the first plurality of acoustic vents and thesecond plurality of acoustic vents do not overlap; and a closingmechanism coupled to the first membrane and the second membrane, theclosing mechanism configured to close an acoustic path through the firstmembrane and the second membrane by moving the first membrane and thesecond membrane into contact during a first mode, wherein the firstplurality of acoustic vents are sealed to the second membrane and thesecond plurality of acoustic vents are sealed to the first membrane whenthe acoustic path is closed, and open the acoustic path by moving thefirst membrane and the second membrane out of contact during a secondmode.
 8. The MEMS device of claim 7, further comprising an actuatingmechanism configured to excite oscillations of the first membrane or thesecond membrane.
 9. The MEMS device of claim 8, wherein the firstmembrane and the second membrane each comprise a deflectable membrane.10. The MEMS device of claim 9, wherein the actuating mechanismcomprises a perforated backplate attached to the support structure andconfigured to generate an electrostatic force between the perforatedbackplate and the first membrane or the second membrane.
 11. The MEMSdevice of claim 9, wherein the actuating mechanism comprises apiezoelectric structure configured to generate a first force on thefirst membrane and a second force on the second membrane.
 12. The MEMSdevice of claim 7, wherein: the first membrane and the second membraneeach comprise a deflectable membrane; and the closing mechanismcomprises a piezoelectric structure configured to generate a first forceon the first membrane and a second force on the second membrane duringthe first mode, the first force and the second force configured to movethe first membrane and the second membrane into contact.
 13. The MEMSdevice of claim 7, wherein the closing mechanism comprises anelectrostatic structure configured to generate an electrostatic forcebetween the first membrane and the second membrane during the firstmode.
 14. A method of operating a MEMS device comprising a firstmembrane attached to a support structure and including a first pluralityof acoustic vents, and a second membrane attached to the supportstructure and including a second plurality of acoustic vents, whereinthe second plurality of acoustic vents are offset from and do notoverlap the first plurality of acoustic vents, the method comprising:closing an acoustic path through the first membrane and the secondmembrane by moving the first membrane and the second membrane intocontact during a first mode, wherein the first plurality of acousticvents are sealed to the second membrane and the second plurality ofacoustic vents are sealed to the first membrane when the acoustic pathis closed; and opening the acoustic path by moving the first membraneand the second membrane out of contact during a second mode.
 15. Themethod of claim 14, wherein the first membrane and the second membraneeach comprise a deflectable membrane.
 16. The method of claim 15,further comprising exciting oscillations of the first membrane and thesecond membrane.
 17. The method of claim 16, wherein exitingoscillations comprises generating an electrostatic force between aperforated backplate and the first membrane and the second membraneusing an actuating mechanism.
 18. The method of claim 16, whereinexiting oscillations comprises using a piezoelectric structure togenerate a first force on the first membrane and a second force on thesecond membrane.
 19. The method of claim 15, wherein closing theacoustic path comprises using a piezoelectric structure to generate afirst force on the first membrane and a second force on the secondmembrane during the first mode, wherein the first force and the secondforce are configured to move the first membrane and the second membraneinto contact.
 20. The method of claim 14, wherein closing the acousticpath comprises generating an electrostatic force between the firstmembrane and the second membrane during the first mode.